1. Field of the Invention
This invention relates to an apparatus for producing hydrogen directly from solar energy. More particularly, this invention relates to direct photoelectrolysis of water to generate hydrogen directly with sunlight employing a combination of advances in fuel cell technology, photovoltaic technology, photoelectrochemistry, and thin film technology.
2. Description of Related Art
Solar hydrogen generation represents an important long-term objective for the energy industry. Hydrogen is an important future energy carrier and energy storage medium. Efficient, low-cost methods of making hydrogen from renewable solar energy are an important element of the future hydrogen economy. With clean and abundant solar energy, solar photovoltaic, photoelectrochemical, or photocatalytic hydrogen generation could become viable technologies. However, to make this a reality, it is necessary to reduce costs, increase efficiency, and improve service life.
For current solar photovoltaic cell-driven electrolysis, the overall efficiency is the product of the efficiency of the solar cell and the efficiency of the electrolyzer. Solar cell efficiencies have been reported from 6% to as high as 32% with different materials. Current electrolyzer efficiency is approximately 75%. Hence solar cell-driven electrolysis efficiency could be from 4.5 to 24%, while in practice values at the low end of this range are encountered. These low efficiencies are in part due to efficiency losses from sunlight absorption by a liquid electrolyte layer, impediments to the departure of product gases from the photoelectrodes due to electrolyte surface tension, and high overpotential of the photoelectrodes. In addition, system life is limited by photocorrosion and electrochemical corrosion of the electrode. Further, the costs of all these devices are too high for wide use.
The current design of photoelectrodes is an additional hindrance to the development of improved photoelectrochemical systems because the semiconductors employed therein are fabricated on conductive substrates. With this type of design, there is no way to reduce the thickness of the electrolyte layer and eliminate the surface tension that acts as an inhibitor to the release of product gases because the reactant water and electrolyte must be transported to the front of the electrode.
Numerous efforts have been made to enhance the efficiency and stability of photoelectrochemical cells. The general approach has been to coat a layer of protective materials, which may be organic substances, active metal ions, noble metals, light sensitive dyes and more stable semiconductors, such as metal oxides, onto the photoelectrode surface. Recent developments include a thin film dye to sensitize the semiconductor electrodes in photoelectrochemical cells. Although the use of light sensitive dyes on the semiconductor electrode surface has improved the light absorption efficiency thereof, it is still necessary that the mass transport rate be increased and that the electrolyte thickness be reduced.